In recent years, a magnetic head and MRAM (magnetoresistive Random Access Memory) using a magneto-resistance element to show GMR (Giant Magneto-Resistance) effect have been developed. The magneto-resistance element used in such devices has a structure called spin valve, and includes an anti-ferromagnetic layer/a ferromagnetic layer/a nonmagnetic layer/a ferromagnetic layer. When the nonmagnetic layer is a conductive layer formed of material such as Cu, the structure is called a spin-valve type GMR, and when the nonmagnetic layer is an insulating layer, the structure is called a spin-valve type TMR (Tunnel Magneto-Resistance).
FIG. 1 shows a sectional view of a spin-valve type TMR magneto-resistance device. With reference to FIG. 1, the magneto-resistance device includes a lower layer 124 as an electrode formed on a substrate 101; an anti-ferromagnetic layer 123; a pinned ferromagnetic layer 120; a tunnel insulating layer 122; a free ferromagnetic layer 12; and a surface layer 125. The pinned ferromagnetic layer 120 has a spontaneous magnetization whose magnetization direction is fixed, and the free ferromagnetic layer 121 has a spontaneous magnetization whose magnetization direction can be inverted. In order to firmly fix the direction of the spontaneous magnetization, the pinned ferromagnetic layer 120 is often formed to be connected to the anti-ferromagnetic layer 123. Consequently, the magnetization is firmly fixed to one direction based on an exchange bias from the anti-ferromagnetic layer 123. Exchange interaction that the anti-ferromagnetic layer 123 provides for the pinned ferromagnetic layer 120, firmly fixes the spontaneous magnetization of the pinned ferromagnetic layer 120. The anti-ferromagnetic layer 123 is generally formed of an anti-ferromagnetic material (Mn-based alloy) containing Mn, such as IrMn and PtMn. Also, the free ferromagnetic layer 121 is often formed of a hard ferromagnetic layer 121b formed of a ferromagnetic material having a high spin polarization rate, and a soft ferromagnetic layer 121a formed of a soft ferromagnetic material. Such a structure of the free ferromagnetic layer 121 makes it possible to facilitate the inversion of the spontaneous magnetization of the free ferromagnetic layer, while increasing a magneto-resistance change rate (a MR rate) of the magnetic tunnel junction. In general, the hard ferromagnetic layer 121b is formed of a ferromagnetic material containing Co, such as Co and CoFe. The soft ferromagnetic layer 121a is generally formed of a ferromagnetic material containing Ni (Ni alloy) such as NiFe, which has small magnetization and soft magnetism. The tunnel insulating layer 122 as a nonmagnetic layer is formed as a very thin insulating film to an extent that tunnel current can flow through it. The tunnel insulating layer 122 is generally formed of an insulator such as AlOx. The lower layer 124 and the surface layer 125 function as electrodes of the magneto-resistance elements.
In the TMR element, the current flows in the vertical direction to the film surface. The magnetization direction of the free ferromagnetic layer is rotated to a magnetic field direction by an external magnetic field, and the resistance of the magneto-resistance element is changed depending on a relative magnetization angle between the free ferromagnetic layer and the pinned ferromagnetic layer.
One of the problems in using the TMR element as an MRAM element is a thermal stability. It is necessary for manufacture of the MRAM to use a process of manufacturing a semiconductor device. For example, in a process of forming a wiring line and an insulating film, and in a thermal treatment process in hydrogen atmosphere for improving the performance of a transistor, it is assumed that the temperature of nearly 400° C. is applied to the TMR element. The conventional TMR element has a low heat resistant temperature of 300° C. Therefore, if these processes are applied as they are to the MRAM including the TMR elements, an elements characteristic is deteriorated. One of the reasons of the deterioration is diffusion between layers of the magneto-resistance elements when the high temperature is applied to the magneto-resistance elements.
As described above, when the conventional TMR element is used as the MRAM element, it is always necessary to consider the thermal treatment temperature in the manufacturing process. Therefore, available device structures and manufacturing processes are limited. In this way, it is demanded to improve a thermal resistance of the TMR element in order to achieve a high-performance MRAM with excellent reliability and to reduce a manufacturing cost. The thermal resistance of up to approximately 400° C. is desired.
For example, in Japanese Laid Open Patent Application (JP-P2000-20922A: a first conventional example), the diffusion between the soft ferromagnetic layer and the hard ferromagnetic layer in a free layer is described. A problem pointed out here is the diffusion of Ni contained in the soft ferromagnetic layer into the hard ferromagnetic layer. The diffusion of Ni into the hard ferromagnetic layer deteriorates the characteristic of the magneto-resistance element. In the first conventional example, an oxide film or a nitride film formed of nonmagnetic elements is provided between the hard ferromagnetic layer and the soft ferromagnetic layer to prevent the interdiffusion.
However, when the hard ferromagnetic layer and the soft ferromagnetic layer are separated by such an oxide film and the like, the magnetic coupling is extremely weakened sometimes since direct exchange interaction between the above two layers is lost. Coercive force of the free ferromagnetic layer becomes large even if the diffusion preventing layer is thinned exceedingly. As a result, steepness in magnetization inversion might be lost.
In Applied Physics Letters (Vol. 78, May 7, 2001, pp. 2911-2913) (the second conventional example), Zongzhi Zhang, et al., report a TMR element that shows a high MR ratio even after a thermal treatment at a high temperature, by forming an oxide layer of ferromagnetic elements such as FeOx, CoFeOx, and the like, between the pinned ferromagnetic layer and the tunnel insulating layer. In this method, however, the oxide layer of the ferromagnetic elements decreases the MR ratio and increases the junction resistance. This is because the oxide of the ferromagnetic element, e.g., CoOx, is an anti-ferromagnetic insulator, which functions as a tunnel barrier of undesirable property that causes a leakage current not depending on a spin, and a spin scattering of tunneling electrons. In addition, CoOx, FeOx, and NiOx, as the oxides of Co, Fe, and Ni have high oxide generation free energy and are instable, and are decomposed in a temperature range of 300° C. to 350° C. Therefore, it is difficult to obtain a thermal resistance of near to 400° C.
In Japanese Laid Open Patent Application (JP-P2001-237471A) (a third conventional example), an oxide magnetic layer is inserted in the pinned ferromagnetic layer and the free ferromagnetic layer in a spin valve type GMR element, in order to improve thermal stability of exchange coupling of the pinned ferromagnetic layer, and to increase the MR ratio through increase in resistance of the magnetic layer. An oxide magnetic layer is used such as Fe3O4 and CoFe2O4 containing iron oxide as a main component and they are added with Si, Al, B, N, Y, and La. In such a spin valve type GMR element that electric current flows in a plane, the high resistance of the pinned ferromagnetic layer prevents the distribution of the electric current out of the conductive layer in the spin valve type GMR element other than the nonmagnetic layer, and the MR ratio can be increased. However, in the TMR element in which the electric current flows in the direction perpendicular to the plane and a magnetoresistance effect of the tunneling current is important, the above-mentioned effect of increasing the MR ratio cannot be expected. Oppositely, in the oxide magnetic layer functions as a series resistance that does not contribute to the tunneling magneto-resistance in the TMR element. Therefore, all the element resistances increase, so that S/N (signal-to-noise) ratio is decreased.
In Japanese Laid Open Patent Application (JP-P2002-158381A) (a forth conventional example), a problem is pointed out that Mn diffuses from the anti-ferromagnetic material containing Mn into the pinned ferromagnetic layer. In the fourth conventional example, the pinned ferromagnetic layer is formed from two ferromagnetic layers and an insulating layer or an amorphous magnetic layer (a diffusion preventing layer) that is provided between the above-mentioned ferromagnetic layers. Thus, Mn is prevented from diffusing into the pinned ferromagnetic layer.
However, in such a diffusion prevention layer, the metal ferromagnetic layer is separated by the diffusion preventing layer, which causes the following problems. When the insulating layer formed of oxide is used as the diffusion preventing layer, the resistance of the diffusion preventing layer is also added in series to the tunneling magneto-resistance, and functions as an additional resistance. In this case, the S/N (signal-to-noise) ratio of the output of the TMR element decreases. In addition, when the insulating layer of the oxide of the non-magnetic element is used as the diffusion preventing layer, the magnetic coupling between the two separated ferromagnetic layers is exceedingly weakens even if the diffusion preventing layer becomes thinner. As a result, the magnetization of the pinned ferromagnetic layer is not fixed to one direction. Even in case that the diffusion preventing layer is the insulating layer that contains the oxide of the ferromagnetic element, only Fe3-XO4 (0<x<⅓) indicates the ferromagnetism, which includes (CoFe2) O4 (Co ferrite), Fe3O4 (magnetait), and γ-Fe2O3 (maghematait) having a spinel structure. The other oxides of ferromagnetic elements are an anti-ferromagnetic material or a paramagnetic material. Moreover, even if the diffusion preventing layer is formed of a spinel oxide ferromagnetic substance, there is a problem of the thermal instability that oxygen decouples easily at a high temperature as mentioned above.
Further, the amorphous magnetic layer is in a non-equilibrium state and tends to be changed generally into a more stable state (e.g., through crystallization, including a material of peripheral film) in application of heat. The tendency greatly depends on the material. Therefore, it cannot be always said that the amorphous magnetic layer itself is effective for the diffusion prevention.
As mentioned in the first to fourth conventional examples, in the magneto-resistance element, in which the oxide layer of the non-magnetic element or the oxide layer of the ferromagnetic element is inserted, there are the problems such as the deterioration of MR characteristic, the low thermal stability, the increase of the resistance through the insertion of the oxide layer, the decrease of the S/N (signal-to-noise) ratio due to the increase of the resistance, and the remarkable decrease of the ferromagnetic coupling between the two ferromagnetic layers separated by the oxide layer. Therefore, it has been necessary to prevent the diffusion and solve these problems at the same.
As the result of examining the thermal deterioration mechanism of the MR element in the thermal treatment at approximately 400° C., the inventors of the present invention found that Ni in the free ferromagnetic layer diffuses into the tunnel insulating layer (tunnel barrier) at the high temperature, and the diffusion of Mn in the anti-ferromagnetic layer into the tunnel insulating layer especially cause of the thermal deterioration. Since Ni and Mn diffuse at a relatively low temperature, the deterioration of the tunnel insulating layer due to the diffusion of Ni and Mn is serious.
Therefore, a technique is demanded that diffusion of Ni in the free ferromagnetic layer into the tunnel insulating layer (tunnel barrier) at the high temperature can be prevented without losing the characteristic of the TMR element in a stable state. Further, a technique is demanded that the diffusion of Mn in the anti-ferromagnetic layer into the tunnel insulating layer can be prevented.
In conjunction with the above-mentioned description, a thin film magnetic head is disclosed in Japanese Laid Open Patent Application (JP-A-Showa 62-132211). In this conventional thin film magnetic head, the change in an applied signal magnetic field is detected as a change in the resistance of a ferromagnetic thin film having one-axis magnetic anisotropy. The thin film magnetic head has the ferromagnetic thin film formed between SiO2 films.
Also, a composite bias magneto-resistance effect head is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 3-268216). In this conventional technique, the composite bias magneto-resistance effect head has a three-layer film of a permalloy thin film as a magneto-resistance effect film formed on substrate, a Nb thin film for shunt bias, and a soft magnetic bias film. A magneto-striction of the permalloy thin film is from +2×10−6 to −2×10−6.
Also, a magneto-resistance element is disclosed in Japanese Laid Open Patent Application (JP-P2002-190631A). In this conventional technique, the magneto-resistance element includes a middle layer and a pair of magnetic layers putting the middle layer therebetween. One of the magnetic layers is a pinned magnetic layer, which is hard to be magnetically inverted against external magnetic field, compared with the other magnetic layer. The pinned magnetic layer is a multi-layered film of at least one non-magnetic layer and magnetic layers putting the non-magnetic substance layer therebetween. The magnetic layers are magnetostatically or anti-ferromagnetically coupled through the non-magnetic substance layer. When the mth magnetic layer (m is an integer more than 0) from a middle layer side is referred to as a magnetic layer m, and the average saturation magnetization and the average film thickness of the magnetic layer m are assumed to be Mm and dm, respectively, 0.5<Mde/Mdo<1 is met, if a total summation of Mm*dm in case that m is an odd number is Mdo, and the total summation of Mm*dm in case that m is an even number is Mde.